Microelectromechanical systems (MEMS) devices are employed as actuators, switches, and sensors (e.g., accelerometers, gyroscopes, pressure sensors, etc.) in a broad range of applications. When utilized as a sensor, a MEMS device commonly functions by sensing changes in capacitance between stationary and movable electrodes, which are spaced apart and interleaved in, for example, a comb-type arrangement. The movable electrodes may be joined to a larger movable structure commonly referred to as a “proof mass,” which is resiliently suspended over an underlying substrate by a number of elongated, flexible beams or “spring members.” Depending upon the particular design of the MEMS device, the proof mass may be movable along a single axis, two perpendicular axes, or three orthogonal axes. During operation of the MEMS device, a voltage differential is applied across the stationary electrodes, the movable electrodes, or a combination thereof. As the proof mass moves in response to acceleration of the MEMS device, the movable electrodes are displaced with respect to the stationary electrodes and the capacitances between the electrodes vary accordingly. By monitoring these capacitances, changes in the acceleration or other inertia-related parameter of the MEMS device can be determined.
When containing moveable structures, a MEMS device may be subject to a fault condition known as “stiction.” Stiction occurs when the movable components of a MEMS device are prevented from returning fully to their original design position after displacement due to an interfering force, such as electrostatic attraction, direct chemical bonding, or capillary forces. Stiction can occur both during in-field usage of a MEMS device and during MEMS fabrication, particularly when a wet etch is utilized to remove sacrificial layers and thereby release the movable MEMS structures. The likelihood of stiction is affected by multiple factors including the physical characteristics of the MEMS device and the environmental conditions (e.g., temperatures and humidity levels) to which the MEMS device is exposed. Several countermeasures have been developed for reducing stiction in MEMS devices, such as the application of low friction coatings over those surfaces prone to stiction. Such stiction countermeasures, however, typically only reduce (rather than eliminate) the likelihood of stiction and can add undesired cost, complexity, and duration to the MEMS manufacturing process. Regardless of whether such countermeasures are employed, it remains desirable to thoroughly and accurately assess the tendency of MEMS devices to suffer from stiction for quality control and fault analysis purposes. In this manner, stiction-prone MEMS devices can be identified and addressed prior to release into the commercial marketplace.
For simplicity and clarity of illustration, descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the exemplary and non-limiting embodiments of the invention described in the subsequent Detailed Description. It should further be understood that features or elements appearing in the accompanying figures are not necessarily drawn to scale unless otherwise stated. For example, the dimensions of certain elements or regions in the figures may be exaggerated relative to other elements or regions to improve understanding of embodiments of the invention.